1. Field of the Invention
The present invention relates to a crystal oscillator of a surface mount type, and more particularly, to a miniature surface mount crystal oscillator which has a crystal blank mounted on an IC (integrated circuit) chip.
2. Description of the Related Art
A crystal oscillator which includes a quartz crystal unit and an oscillation circuit using the crystal unit integrated therein is used as a reference source for the frequency and time in a variety of devices. Particularly, surface mount crystal oscillators are contained in portable electronic devices as a reference source for the frequency and time because of its small size and light weight. In recent years, the oscillation frequency of surface mount crystal oscillators is made increasingly higher, driven by ever widespreading optical communications systems as well, to reach even into a 600 MHz band.
A further reduction in size as well as a higher oscillation frequency have been required for surface mount crystal oscillators. Generally, a surface mount crystal oscillator comprises a crystal blank as a crystal unit, and an IC chip which has integrated therein an oscillation circuit that uses the crystal unit, and these components are encapsulated in a surface mount package. Each of Japanese Patent Laid-open application No. 11-145728 (JP, 11-145728A), Japanese Patent Laid-open application No. 2000-196360 (JP, P2000-196360A) and Japanese Patent Laid-open application No. 2001-28516 (JP, P2001-28516A) each discloses a reduction in size of a surface mount crystal oscillator by mounting a crystal blank on an IC chip, particularly, a reduction in planar dimensions on a wiring board on which the crystal oscillator is mounted.
FIG. 1 illustrates a conventional surface mount crystal oscillator which has a crystal unit on an IC chip.
The crystal oscillator illustrated in FIG. 1 comprises package body 3 having a recess formed on one surface thereof; IC chip 1 and crystal blank 2 accommodated in the recess; and cover 4 which is put on the package body 1 to hermetically encapsulate IC chip 1 and crystal blank 2 in package body 3. Here, crystal blank 2 is mounted on IC chip 1. IC chip 1 has an oscillation circuit, not shown, using crystal blank 2, and the like integrated therein, and is fabricated by a normal semiconductor device manufacturing process. In IC chip 1, circuits such as the oscillation circuit are disposed on a surface of a semiconductor substrate, which constitutes IC chip 1, on the upper side as viewed in the figure. Therefore, out of both principal surfaces of IC chip 1, the surface of the semiconductor substrate formed with the oscillation circuit and the like is called the “circuit forming surface.”
As illustrated in FIG. 2, a plurality of IC terminals 5 are formed to oppose each other, including a power supply terminal, a ground terminal, and an output terminal, along the periphery of the circuit forming surface of IC chip 1. Further, a pair of crystal connection terminals 5a, 5b are disposed at positions near the center of the circuit forming surface for electrically connecting crystal blank 2 to the oscillation circuit. The circuit forming surface of IC chip 1 is formed with an oxide film (i.e., insulating layer), not shown, made, for example, of SiO2 on which IC terminals 5 and crystal connecting terminals 5a, 5b are formed using Al (aluminum), Au (gold) or the like.
Crystal blank 2, which is an AT-cut quartz crystal blank, by way of example, is generally formed in a rectangular shape as illustrated in FIG. 3. In the crystal oscillator described herein, crystal blank 2 has a plane geometry smaller than the geometry of IC chip 1, so that IC terminals 5 are not covered with crystal blank 2.
Excitation electrodes 6a, 6b are disposed on both principal surfaces of crystal blank 2, respectively. From these excitation electrodes 6a, 6b, extension electrodes 7a, 7b extend to positions near opposite corners of one shorter side of crystal blank 2. Each extension electrode 7a, 7b is folded back to the opposite principal surface on one edge portion of crystal blank 2. Then, these extension electrodes 7a, 7b are secured to crystal connection terminals 5a, 5b, respectively, disposed on the circuit forming surface of IC chip 1 for electrical and mechanical connection therebetween by ultrasonic thermo-compression bonding using bumps 8 made of Au or the like, or by thermo-compression bonding using an eutectic alloy. In this way, crystal blank 2 is held above IC chip 1 to be in parallel with the circuit forming surface of IC chip 1. As the eutectic alloy used in the thermo-compression bonding, a gold-germanium (AuGe) alloy is used, by way of example.
Package body 3, which is made, for example, of laminate ceramics, is formed with a step in the inner wall of the recess. Mounting electrodes 10 are disposed on the outer surface of package body 3 for use in mounting the crystal oscillator on a wiring board. Circuit terminals 9 are disposed on the top surface of the step in the recess so as to correspond to IC terminals 5 of IC chip 1, respectively, and these circuit terminals 9 are electrically connected to mounting electrodes 10, respectively, through a lamination plane of the laminate ceramics. IC chip 1 has the surface opposite to the circuit forming surface, secured to the bottom of the recess in package body 3. Then, IC terminals 5 on the circuit forming surface are electrically connected to circuit terminals 9 by wire bonding using gold wires 11 or the like. In this way, mounting terminals 10 on the outer surface of package body 1 are electrically connected to the ground terminal, power supply terminal, and output terminal of IC chip 1.
Such a surface mount crystal oscillator can be reduced in height and plane geometry because crystal blank 2 smaller than IC chip 1 is directly secured onto IC chip 1 for integration. Also, since crystal blank 2 is secured to IC chip 1 using a metal without using an organic conductive adhesive, the resulting crystal oscillator is not affected by a gas generated from the conductive adhesive, and therefore exhibits good aging characteristics.
FIG. 4 illustrates a conventional surface mount crystal oscillator which is further reduced in size. In the crystal oscillator illustrated in FIG. 4, an IC chip is electrically connected to a package body by ultrasonic thermo-compression bonding using bumps, instead of wire bonding. This crystal oscillator employs surface mount package body 3A which has a recess, and IC chip 1A having an integrated oscillation circuit and the like is disposed in the recess such that its circuit forming surface opposes the bottom of the recess in package body 3A. Crystal blank 2 is disposed on the back side of IC chip 1A, i.e., on the principal surface which is not the circuit forming surface. Then, the recess is covered with cover 4 to hermetically encapsulate IC chip 1A and crystal blank 2 within package body 3.
As illustrated in FIG. 5, a plurality of IC terminals 5 are formed, including a power supply terminal, ground terminal, and output terminal connected to an oscillation circuit, along the periphery of the circuit forming surface of IC chip 1A in a manner similar to the foregoing. A plurality of circuit terminal 9 are disposed, respectively, corresponding to IC terminals 5, on the bottom of the recess in package body 3A, and IC terminals 5 are secured to circuit terminals 9 for electrical connection by ultrasonic thermo-compression bonding using bumps made of Au (gold) or the like.
Package body 3A, which is made, for example, of laminate ceramics, is not formed with a step in the recess. Mounting electrodes 10 are disposed on the outer surface of package body 3A in a manner similar to the foregoing, such that mounting electrodes 10 are electrically connected to circuit terminals 9 through a lamination plane of the laminate ceramics.
A pair of crystal connection terminals 5a, 5b are disposed on the back side of IC chip 1A for connection to crystal blank 2, as illustrated in FIG. 6. A pair of auxiliary terminals 15a, 15b are formed on the circuit forming surface, corresponding to crystal connection terminals 5a, 5b. Crystal connection terminals 5a, 5b are electrically connected to auxiliary terminals 15a, 15b by through-holes (electrode through-holes) 16 which extend through IC chip 1A. Since auxiliary terminals 15a, 15b are electrically connected to the oscillation circuit on the circuit forming surface, crystal connection terminals 5a, 5b are also electrically connected to the oscillation circuit. The back side of IC chip 1A is formed with an oxide film (i.e., insulating layer), not shown, made, for example, of SiO2, and crystal connection terminals 5a, 5b are formed on the oxide film by Al (aluminum), Au (gold) or the like.
As crystal blank 2, one similar to that illustrated in FIG. 3 can be used. Then, crystal blank 2 has its extension electrodes 7a, 7b secured to crystal connection terminals 5a, 5b for electrical and mechanical connection therebetween, for example, by thermo-compression bonding using an eutectic alloy such as AuGe or by ultrasonic thermo-compression bonding using bumps made of Au or the like, such that crystal blank 2 is held above IC chip 1A so as to be in parallel with the back side of IC chip 1A.
Since the surface mount crystal oscillator illustrated in FIG. 4 does not require a space for wire bonding and has crystal blank 2 directly secured onto IC chip 1A for integration, the surface mount crystal oscillator can be further reduced in height and plane geometry. Since no conductive adhesive is used, the resulting crystal oscillator is not affected by a gas generated from the conductive adhesive and therefore exhibits good aging characteristics.
In recent years, crystal oscillators have been required to have higher oscillation frequencies. With an AT-cut quartz crystal blank, its resonance frequency is inversely proportional to its thickness. For example, an AT-cut crystal blank having a resonance frequency of 100 MHz has a thickness of approximately 16.7 μm in a vibration region. A crystal blank in a 622-MHz band for use as optical communications has a thickness of approximately 2.2 μm in a vibration region. Thus, one of principal surfaces of crystal blank 2 is formed with depressed portion 11 by etching or the like to define vibration region 2A therein, as illustrated in FIG. 7, in order to increase the resonance frequency while maintaining the mechanical strength of the crystal blank. In this crystal blank, the resonance frequency is increased by reducing the thickness of the crystal blank in the depressed portion, i.e., vibration region, and the mechanical strength is maintained by holding vibration region 2A by relatively thicker portion 2B around the depressed portion. Further, Japanese Patent Laid-open application No. 2004-40693 (JP, P2004-40693A) discloses a crystal unit having a high vibration frequency which includes a first crystal plate having a through-hole and a second crystal plate having a flat shape adhered to each other by direct bonding, where a vibration region is defined at the position of the through-hole of the first crystal plate.
In the surface mount crystal oscillators illustrated in FIGS. 1 and 4, the crystal blank is secured to the IC chip using bumps, eutectic alloy or the like on both sides on one edge of the crystal blank from which the extension electrodes extend, i.e., at two positions in a peripheral region. Since the bumps and eutectic alloy are made of metal and fairly hard, a stress acts between the two positions in the peripheral region, for example, due to a difference in coefficient of thermal expansion between the IC chip and the crystal blank to distort the crystal blank. The oscillation characteristics of the crystal blank exacerbate due to such a stress applied thereto. Particularly, the problem of the exacerbated oscillation characteristics due to the applied stress is prominent in high-frequency crystal oscillators because such crystal oscillators employ a crystal blank having a smaller thickness.